Processes for the removal of carbon dioxide (CO.sub.2) from industrial gases (e.g. natural gas, refinery gas, and certain synthetic gases) and the like are well known to the commercial petroleum and chemical industries. Likewise, numerous absorbents, generally aqueous based, have been employed in the gas-liquid processes of industry. Among the more widely employed solvents are the alkanolamines (e.g., monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA)), sodium carbonate, sulfolane and sulfolanediisopropanolamine (Shell's Sulfinol process) which are corrosive to metals, particularly at temperatures of regeneration used in most aqueous absorbent gas purification and recovery processes. These absorbents are also subject to thermal degradation particularly in the presence of metals. The degradation products of both metal corrosion and degradation, as well as the acid gases absorbed, accelerate both corrosion and absorbent degradation.
A less well known, yet commercially practiced application of gas treating (absorbing), is the removal of CO.sub.2 from gas streams, e.g. flue gas, containing oxygen from a few parts per million to about two percent. The degradation occurs as described in U.S. Pat. No. 3,137,654 which reports even small amounts of oxygen cause deterioration of ethanolamine and diethanolamine. Corrosion likewise accompanies these applications due in part to the types of metals used in commercial plants.
The commercial scale operations, carried out today usually employs absorbents such as aqueous alkanolamine solutions having amine concentrations from 10 to about 20 percent. These processes are commonly sized such that the solvent circulates at rates to absorb (load) acid gases into the solution at about 20 to 40 percent of its theoretical capacity when the oxygen content of the gas being treated is only a few parts per million.
The lesser known commercial application, CO.sub.2 removal from gas streams containing a few parts to about two percent oxygen, or air as in U.S. Pat. No. 3,137,654, utilize alkanolamine concentrations ranging from 7 to 12 percent and in some exceptional cases as high as 24 percent (the concentration of a 4 N amine solution reported in U.S. Pat. No. 3,137,654)
While many of these commercial processes use various additives to abate both corrosion and degradation, and most inhibitor formulations are based on patent and published literature technology, which allude to and even claim utility in the presence of oxygen, the experience in the field has been conflicting at best. Commercial proceses audited and/or reported in the literature routinely limit the amount of oxygen (in the presence of CO.sub.2) to only a few percent. Generally the plants are designed for maximum loadings of less than 50 percent of theoretical solvent capacity while employing 10 to 20 percent amine concentrations of the aqueous absorbent solutions to control corrosion and solvent degradation. However, considerable degradation as well as poor corrosion inhibition profiles are still obtained.
In some instances activated carbon absorbers are used in an effort to remove the degradation products and corrosion products. However, in the instance of CO.sub.2 removal, U.S. Pat. No. 3,137,654 teaches that activated carbon filters in the amine circuit enhance the degradation rate of MEA rather than reduce the effect.
In an industry faced with ever increasing fuel and construction costs, the desire to employ higher loadings and stronger absorbent concentrations is natural. However, many plants are not converting because of the increased corrosion and degradation problems encountered with higher loadings and/or higher absorbent concentrations. Several mild excursions above conventional concentrations and/or loadings on a commercial scale have been made with very poor results.
It would, therefore, be advantageous to find an inhibitor or inhibitor mixture capable of maintaining or reducing corrosion and/or degradation rates while employing loadings in excess of 50% of theoretical and absorbent concentrations in the 30 to 40 percent range.